ATLANTA — Former President Jimmy Carter announced Thursday that his cancer is on four small spots on his brain and he will immediately begin radiation treatment, saying he is ‘‘at ease with whatever comes.’’

‘‘I’m ready for anything and looking forward to a new adventure,’’ said Carter, appearing upbeat and making jokes as he openly talked about his cancer during a news conference.

So far, the pain has been ‘‘very slight’’ and Carter said he hasn’t felt any weakness or debility. Still, he will cut back on his work with the Carter Center and will give the treatment regimen his ‘‘top priority.’’ His first radiation treatment was set for Thursday afternoon.

Carter, appearing in a dark blazer, red tie and jeans and surrounded by friends and family, said at first he thought the cancer was confined to his liver and that the operation earlier this month had completely removed it, ‘‘so I was quite relieved.’’

But that same afternoon, an MRI showed it was on his brain.

‘‘I just thought I had a few weeks left, but I was surprisingly at ease. I’ve had a wonderful life,’’ the 90-year-old Carter said.

He didn’t give any prognosis, but spoke about receiving treatments for at least several months and left open the possibility of traveling to Nepal in November.